Such an environmental cell and the method for using such a cell in a Scanning Electron Microscope (SEM) are known from “A simple low-vacuum environmental cell”, M. E. Ervin, Microsc. Microanal. 9, 18-28, 2003.
In a SEM a finely focused beam of primary electrons with a fixed energy of e.g. between 1 keV and 30 keV is scanned over the surface of a sample. As a result of the primary electrons impinging on the sample, secondary radiation is emitted by the sample. Said secondary radiation comprises light, X-rays, secondary electrons and back-scattered electrons. The difference between secondary electrons and back-scattered electrons is the energy with which they escape from the sample: while secondary electrons have an energy of up to approximately 10 eV, back-scattered electrons retain a large portion of the energy with which the primary electrons impinge on the sample. By detecting part of the secondary radiation, e.g. the back-scattered electrons and/or the secondary electrons, an image of the sample can be constructed.
To direct the electrons to the sample implies that most of the electrons may not interact with gas atoms or molecules, as this would scatter these electrons away from the beam of primary electrons and would cause these scattered electrons to impinge elsewhere on the sample. Therefore the sample is placed in an evacuated volume with a pressure of typically less than 10−3 mbar.
A problem arising from the irradiation of the sample with a beam of electrons is that the sample may become charged. Said charging may influence the position where the beam impinges on the sample, as well as the detection of the secondary electrons and back-scattered electrons. It is known that said charging can be eliminated, or at least strongly reduced, by placing the sample in a volume with a pressure between 0.1 and 10 mbar: some of the atoms or molecules in the gas will become ionized by interaction with primary of secondary radiation, and these ionized atoms or molecules will drift to the charged parts of the sample and neutralize said charge. However, a draw-back of this is that sufficient ionization must take place and therefore the electrons causing this ionization are scattered out of the primary beam, resulting in other parts of the sample being irradiated as well. As described by C. Mathieu, “The beam-gas and signal-gas interactions in the variable pressure scanning electron microscope”, Scanning Microscopy Vol. 13, No. 1, pages 23-41, this so-named ‘skirting effect’ results in a reduced signal-to-noise ratio when detecting e.g. the back-scattered electrons.
The known environmental cell comprises a small volume in which a sample is placed. The environmental cell is placed in the SEM. The small volume is surrounded by a vacuum wall which shows a diaphragm facing the SEM column. The diaphragm shows an aperture through which the electron beam may enter the small volume and thereby irradiate the sample. The aperture is sufficiently small to limit the amount of gas escaping from the small volume to an amount that is acceptable to the SEM's vacuum system. Hereby it is possible to have a pressure of e.g. between 0.1 and 10 mbar at the inside of the environmental cell and a much lower pressure between the aperture and the SEM column Charging of the sample is thus avoided or at least greatly reduced while the length over which the primary beam interacts with the gas is limited to the distance between aperture and sample. The volume being small, this distance is equally small, resulting in a small number of electrons scattered by the gas.
A problem arises when detecting the secondary radiation using such an environmental cell: only radiation passing through the aperture can be detected outside the environmental cell. Therefore the known environmental cell is constructed such that the diaphragm is electrically isolated from the rest of the environmental cell. By applying a voltage difference between the rest of the environmental cell (and the sample placed on said rest of the environmental cell) and the diaphragm an electric field is formed between the sample and the diaphragm. As a result electrons coming from the sample form a cascade of ionizations, the so-named gas-cascade. The resultant gas-cascade current is collected by the diaphragm and measured with a sensitive electrometer connected to said diaphragm.
A disadvantage mentioned by Ervin is that the gas-cascade current is quite small (typically 5 nA) and to measure such a small current, the time constant requires that a slow scan rate be used. This implies that focusing the sample and obtaining an image of the sample take much longer than when imaging at e.g. video rate.
Another disadvantage is that by electrically insulating the diaphragm and electrically connecting it with an electrometer, the environmental cell becomes complicated.
Still another disadvantage is that the electrical connection between the diaphragm and the electrometer is likely to pick up interference, resulting in a deterioration of the image and further adding to design constrains.
Yet another disadvantage is that the multiplication of the gas-cascade not only depends on the electric field between sample and diaphragm, but also strongly depends on the pressure of the gas and the composition of the gas: if the pressure is too high, the electrons will not pick-up sufficient energy between collisions to cause ionization, if the pressure is too low an insufficient number of collisions occur.